180 XIII INTEGRATION (b) The function g is measurable, and for each compact set K c X we have I * \d\9vi dfj,< +00. (c) For each function h e Jfc(X), the function gh is integrable. Since we may write g — g± 4- ig2 > where gl and g2 are K- valued, and then 9i =0i -9l> 92=92 ~92, we reduce straightaway ((13.9.6) and (13.10)) to the case where g is a mapping ^0 of X into R. To show that (a) implies (b), cover K by a finite number of open sets V,- such that g<pVj is integrable for each y. Then sup(#<pv.,) is integrable (13.7.4) and hence so is (13.9.14). Hence (b) follows from (13.9.13). To show that (b) implies (c), observe first that g is almost everywhere finite (X being a denumerable union of compact sets), hence gh is measurable (18.104.22.168). Moreover, if L = Supp(/z), we have \gh\ ^ \g<pi\ • \\h\\, hence it follows from (b) and (13.9.13) that gh is integrable. Finally, to show that (c) implies (a), consider a compact neighborhood V of x e X and a continuous mapping h : X -» [0, 1] which is equal to 1 on V and has compact support ((3.18.2) and (4.5.2)). By hypothesis gh is integrable, hence so is gcpv = (gh)(pv (13.9.14). When the equivalent conditions of (13.13.1) are satisfied, the function g is said to be locally integrable (with respect to ju) or locally fji-integrable. Clearly every integrable function is locally integrable. Every measurable function whose restriction to every compact subset of X is bounded almost everywhere (in particular, every function belonging to & £ or JSf £) is locally integrable. Every function belonging to :$?£ or jSf £ is locally integrable, by (13.11.7). Every function equivalent to a locally integrable function is locally integrable. We have remarked in the course of the proof of (13.13.1) that every locally integrable function is finite almost everywhere. The function on R which is equal to l/\x\ when x ^ 0 and is 0 when x = 0 is lower semi- continuous but not locally integrable with respect to Lebesgue measure. For a complex- valued function g to be locally integrable, it is necessary and sufficient that 0tg and Jg should be locally integrable. For a real-valued function g to be locally integrable, it is necessary and sufficient that g+ and g~~ should be locally integrable. Let g be a locally ^-integrable function. Since /H* \fg d^ is defined on the whole of Jfc(X), it is a linear form on this complex vector space. Moreoverne or other of the partitions V u~J(oc).